In the first release of the LTE specifications (Rel-8), a single carrier of up to 20 MHz is used. A control signalling message on the Physical Downlink Control Channel (PDCCH) to signal allocations of transmission resources on either uplink or downlink. In UMTS LTE the downlink control channel PDCCH (Physical Downlink Control Channel) carries information such as resource allocation for uplink or downlink transmission. A PDCCH message can use 1, 2, 4 or 8 Channel Control Elements (CCEs or resource elements)—referred to as CCE aggregation levels 1, 2, 4 or 8.
A mobile station, like a UE in LTE, does not know in advance the location in CCE space of messages intended for it. In principle, the mobile station could attempt to blindly decode all the possible PDCCHs with different starting positions in the CCE space and thus receive any messages intended for that mobile station. However, if the CCE space is large the processing complexity is prohibitive. Therefore a more limited search is configured which consists of a number of search spaces.
A search space is a set of aggregated CCEs (with a certain aggregation level) within which a mobile station (or user equipment (UE) or secondary station) performs blind decoding of all PDCCH payloads possible for that aggregation level. Search spaces are defined per aggregation level; a secondary station thus can have up to four search spaces. For example, the search space of a UE for aggregation level 1 (referred to as 1-CCE) could consist of the CCEs indexed 3, 4, 5, 6, 7, 8, while its search space for aggregation level 8 could consist of the two resource sets of aggregated CCEs consisting of the CCEs indexed by 1, 2, . . . 8 and 9, 10, . . . , 16, respectively. In this example, the UE thus performs six blind decodings for 1-CCEs and two blind decodings for 8-CCEs.
The LTE specification currently requires the UE to perform the following:                6 decoding attempts of 1-CCE aggregation        6 decoding attempts of 2-CCE aggregation        2 decoding attempts of 4-CCE aggregation        2 decoding attempts of 8-CCE aggregation        
The larger aggregations are intended to be used for large messages, and/or small messages when a lower code rate is required, for example under bad channel conditions. However, restricting the search spaces to reduce processing complexity limits the availability of suitable aggregations for different conditions as conditions vary.
In LTE-Advanced, this principle is extended to multiple carriers, and PDCCH signalling will be needed to indicate resource allocations on each of the component carriers (CCs). One PDCCH may indicate an allocation on the same CC or a different CC. It is agreed in RAN2 that a set of CCs may be configured, and a subset of these CCs may be “active”. However, in general it is desirable to avoid having a large number of blind decodes required. This could be achieved by reducing the size of the search space on at least some CCs. In addition, where there is not data for a given UE, DRX (Discontinuous Reception) may be applied. Currently in RAN2 it is agreed that a common DRX cycle would apply to all (active) CCs.
However, further power/blind decoding savings could be achieved by only monitoring a subset of active carriers at the end of each DRX cycle. When data is actually received, more carriers would be monitored. This could be viewed as a special feature of DRX or as dynamically modifying the set of active carriers.
A further proposal under consideration in 3GPP is for one carrier to have special status (e.g. so called “anchor carrier”). Some possible distinguishing features for the anchor carrier could be:                Common search space is monitored for broadcast control messages (not on other CCs)        Full UE specific search space is applied (smaller search space on other CCs)        Only PDCCH on the anchor carrier is monitored at the end of the DRX cycle        
A problem with defining one carrier as the “anchor carrier” is that this carrier may suddenly become unavailable, for example due to a change in interference or channel conditions.
A similar problem may arise with individual component carriers (i.e. one or more may temporarily become of low quality or unavailable).